(This may be slightly off-topic here.) Manna & Pnueli typeset their books in (La)TeX using the font ZMSY10. Here's the head of the metafont file zmsy10.mf that I'm hesitant to publish in full because I'm unsure about its copyright situation:
% Font parameters
% This is the font ZMSY10, containing special symbols used by Zohar Manna.
% It has been ...
I don't know if there is a correct or recommended usage of Unicode codepoints for the LTL operators, but the mapping we used in LaTeX was the following:
□ (U+25A1) => \Box
◇ (U+25C7) => \Diamond
○ (U+25CB) => \medcirc (from txfonts/pxfonts)